Heat treatment boat

ABSTRACT

A heat treatment boat for mounting a number of disc-shaped objects to be treated at a vertical interval for heat treatment thereof in a vertical heat treatment furnace comprises arcuate or ring-shaped support members provided on support rods at a vertical interval for supporting the objects to be treated in surface contact with the undersides of peripheral parts of the objects to be treated. The heat treatment boat is disposed on a ring-shaped intermediate member of high radiant heat absorption disposed on a heat insulating cylinder. The heat treatment boat comprises support rods which are planted on an annular support member circumferentially in accordance with a contour of the objects to be treated and whose upper ends are secured to an annular fixation member. The heat treatment boat of such constitution can reduce occurrences of slips in the disc-shaped objects when heat treated.

BACKGROUND OF THE INVENTION

The present invention relates to a heat treatment boat for heat treatingdisk-shaped objects, such as semiconductor wafers.

Semiconductor wafer (hereinafter called "wafer") preparation includes,as preparation steps, heat treatments at high temperatures for formationof oxide films, diffusion of dopants, etc. As apparatuses for such heattreatments horizontal heat treatment furnaces have been dominant, butrecently vertical heat treatment furnaces are increasingly used becausethey draw in less outside atmosphere.

A vertical heat treatment apparatus using a vertical heat treatmentfurnace uses a vertical heat treatment boat (also called "wafer boat")for mounting a number of wafers at a set vertical interval toload/unload the wafers into the heat treatment furnace.

FIG. 18 shows a conventional heat treatment wafer boat. The conventionalheat treatment wafer boat 1 includes four support rods of 13-16 of,e.g., quartz erected between a disk-shaped top plate 11 and adisk-shaped bottom plate 12. Two (13, 14) of the support rodsrespectively support the wafers at the front left and right in thedirection of advance of the wafers W as viewed in FIG. 18, and the othertwo (15, 16) of the support rods support the wafers at the rear left andright in the direction of advance of the wafers as viewed in FIG. 18.The wafer boat 1 is provided on a heat insulation cylinder 2 which isformed of a heat insulating material.

As shown in FIG. 19, each support rod 13-16 has grooves 17 with avertical width which is a little larger than a thickness of the wafers Wso that the respective grooves support the respective wafers W at theundersides of the peripheral portions. The wafers W are put in/out ofthe grooves 17 by a transfer arm 21 in the direction of the arrowbetween the two front support rods 13, 14.

Such structure of the heat treatment boat 1 has been used inconventional horizontal heat treatment furnaces and is used as it is inthe vertical heat treatment furnaces. That is, in the horizontal heattreatment furnace, the wafers are transferred by a push-up mechanism forpushing up the wafers from below the heat treatment boat and a mechanismfor holding the lifted wafers W. Such transfer has determined thestructure of the heat treatment boat.

In the heat treatment boat 1 of FIG. 18, when a set number of wafers Wto be treated are mounted on the boat 1, an elevator 22 is lifted toload the boat into the heat treatment furnace, so that the wafers W areloaded in the heat treatment furnace to be subjected to a required heattreatment.

In a heat treatment for processing wafers, e.g., to diffuse dopant ionsimplanted in the wafers down to a set depth, the wafers are heated at ahigh temperature of about 1200° C. for a long period of time. In thiscase, the substrate material of the wafers is silicon, since the meltingpoint of silicon is 1410° C., the silicon wafers have an extremely lowyield stress at 1200° C.

On the other hand, there has been a tendency toward increased waferdiameter. Their size has increased from 6 inches to 8 inches, andfurther 12 inch-diameter wafers are being studied. In wafers of suchlarge diameters, when the wafers are heat treated at a temperature neara melting point of their substrate material, crystal deformations calledslip tend to occur near portions of the wafers at which the wafers aresupported by the support rods of the heat treatment boat. These slipsare fine faults which are invisible but can be seen by magnifyingglasses, microscopes. etc.

As shown in FIG. 18, the top and the bottom plates 11, 12 of the waferboat 1 are heated by a heater in the furnace, and because of no heatersabove and below the wafer boat 1 heat is radiated as secondary radiantheat from the top and the bottom plates 11, 12. Since the top and thebottom plates 11, 12 are plane bodies, amounts of the radiant heat areconsiderably large. Accordingly heat is conducted from the support rods11-14 of the wafer boat to the top and the bottom plates 11, 12, i.e.,from the wafers W to the support rods to the top and the bottom plates11, 12, temperature gradients occur in the peripheral portions of thewafers W.

To consider the occurrences of the slips, the micro-areas of the wafersin actual contact with the grooves 17 as shown in FIG. 20 are noted.Large stresses will be applied to these micro-areas as described above,and large temperature differences will take place there, so that largethermal stresses will be applied thereto, and planes of the wafers Wwill be displaced past each other from the bottom to the top of thewafers W, causing the slips. It is considered that larger calories flowespecially from those of the wafers nearer to the top and the bottomends of the wafer boat 1 to the top and the bottom ends thereof, so thatthe slips tend to occur in those wafers W. When the slips occur in theregions of the wafers W for the devices to be formed in, the deviceswill be defective with low yields.

Causes for the slips will be 1) internal stresses due to an own weightof the wafers W, and 2) thermal strain stresses due to disuniformtemperatures in plane of the wafers W. In connection with the cause 1),the support positions by the heat treatment boat 1 are located on theperipheral parts of the wafers and partially at four positions, so thatlarge internal stresses will occur due to an own weight of the wafersnear the supported parts, and the slips will occur when the internalstresses exceed a certain magnitude. In addition, wafers havesoris(bendings) not only within the allowed range, but also due totemperature distributions when heated. Furthermore, a width of thegrooves in the support rods have manufacturing errors, If the wafersshould be apart from any one of the four support positions because ofany of these causes, the wafers would be supported only at three of thefour support positions, so that, as seen from the layout of the supportrods 13-16, a load at the respective support positions will becomeimbalanced, with the result a large stress exceeding a threshold for theoccurrence of the slips will take place at one of the four supportpositions.

As regards the cause 2) above, when the wafers are heated, heat issupplied and removed through the support rods of the heat treatment, sothat temperature differences take place between the central parts of thewafers and the peripheral parts thereof, and thermal strain stressesoccur. The slips will occur when the thermal strain stresses exceed acertain magnitude.

Thus, in heat treating wafers, especially in heat treating wafers at ahigh temperature near a melting point of the substrate of the wafers, asthe wafers have larger diameters, the problem of the slips take place.This problem is a serious obstacle to increasing diameters of wafers.

A further cause which is considered for causing the slips is thermalstrain stresses due to disuniform temperatures in plane of wafers. Withreference to FIG. 21, heat is supplied and removed through support rods3A of a wafer boat 3 when wafers W are heated. Accordingly temperaturedifferences take place between the central parts of the wafers and theperipheral parts thereof, with a result of occurrence of thermal strainstresses. The slips will occur when the thermal strain stresses exceed acertain magnitude. The slips tend to occur especially in lower part ofthe wafer boat 3.

The reason for this will be as follows. In a case that a heat treatingtemperature is above, e.g., 1000° C., the wafer boat 3 is loaded into areaction tube 1 which has been pre-heated up to about 800° C., and whenthe wafers W are heated up to an atmospheric temperature, thetemperature in the reaction tube 1 is raised up to a set heat treatingtemperature. Quartz forming the reaction tube 1 has the low heatconductivity and has good properties as a heat insulating material. Onthe other hand, because of its high light transmittance, quartz does notmuch absorb radiant heat and is slow to be heated. Accordingly a heatinsulator 2 is behind the wafer boat to be heated until the wafer boat 3is loaded in the reaction vessel 1 and has a stabilized atmospherictemperature and until the interior of the reaction tube 1 is heated upto a set temperature, so that heat escapes due to a temperaturedifference between the two from the lower part of the wafer boat 3 tothe heat insulator 2, and a temperature of the lower part of the waferboat 3 cannot be easily raised. As a result, temperature differencesoccur between parts of the wafers W supported by the wafer boat 3, andthe rest parts of the wafers W, and the slip will easily take place.

SUMMARY OF THE INVENTION

In view of the above-described circumstances, the present invention wasmade, and an object of the present invention is to provide a heattreatment boat which can reduce occurrences of slips in heat treatingdisc-shaped objects.

The present invention relates to a heat treatment boat for mounting anumber of disc-shaped objects to be treated at a certain verticalinterval for heat treating the object to be treated in a vertical heattreatment furnace in which a number of arcuate or ring-shaped supportmembers are provided on support rods at a vertical interval, the supportmembers being formed of the same material as the objects to be treated,and supporting the objects to be treated in surface contact with theundersides of peripheral parts of the objects to be treated.

The present invention also relates to a heat treatment boat for mountinga number of disc-shaped objects to be treated at a vertical interval forheat treating the objects to be treated in a vertical heat treatmentfurnace in which a number of arcuate or ring-shaped support members forsupporting the objects to be treated in surface contact with theundersides of peripheral parts of the objects to be treated areremovably provided on support rods at a vertical interval.

According to the present invention, the objects to be treated (e.g.,wafers) are supported with outer peripheral parts thereof in surfacecontact with the support members. Accordingly internal stresses near thesupported parts due to the object's own weight are more mitigated thanthose in the conventional four point support, and even when an object tobe treated has sori (bending), because of the large supported area nolarge load is applied to one part. Furthermore, disuniform temperaturedistributions along the outer peripheries of the objects to be treatedare mitigated. Since the support members are formed of the same materialas the objects to be treated, temperature rising and falling rates in aheat treatment are equal. Accordingly temperature differences in planeare small, and thermal strain stresses are considerably mitigated.

The support members are removable from the support rods. Thisfacilitates their fabrication in comparison with one-piece type. Bypreparing various types of support members suitable for transfer modesof the objects to be treated, when a transfer mode is changed, asuitable type of support members can be selected.

The present invention relates to a heat treatment boat which is disposedon a heat insulating unit of a heat insulating material and includes anumber of plate-type objects to be treated provided at a verticalinterval, the heat treatment boat being loaded from below into avertical reaction tube forming a heat treatment region, the heattreatment boat being disposed on the heat insulating unit through anintermediate member of high radiant heat absorption.

In this structure, when the heat treatment boat is loaded into areaction tube and heated, the intermediate member, whose radiant heatabsorption is high, quickly raises temperatures. Since the lower end ofthe heat treatment boat is in contact with the intermediate member orconnected in one-piece, temperature differences between the lower partof the heat treatment boat and the intermediate member are small.Accordingly radiation from the lower part of the heat treatment boat tothe heat insulating unit is suppressed, with a result that temperaturedisuniformity in the plane of the objects to be treated is mitigated,and occurrence of slips is suppressed.

The present invention relates to a heat treatment boat for supporting anumber of objects to be treated at a vertical interval along a pluralityof vertically extended support rods, the heat treatment boat comprisingan annular support member for supporting lower ends of the support rods,and an annular fixation member for positioning the upper ends of thesupport rods with each other.

In this structure, when objects to be treated are mounted on the heattreatment boat and loaded in a reaction tube, and the interior of thereaction tube is heated secondary radiation take place on the upper andlower ends of the heat treatment boat. But the support members and thefixation members provided on the upper end and the lower end areannular, and radiated calory is small. Calories conducted to the upperends and the lower ends of the support rods are accordingly small, andcalories conducted from the peripheral parts of the objects to betreated to the support rods are small. As results, temperatureuniformity in plane of the objects to be treated is high, andaccordingly thermal stresses in the peripheral parts are small, andoccurrence of slips can be suppressed. The heat treatment boat issupported on a heat insulating unit through a plurality of islands, or aplurality of islands are provided between the heat insulating unit andthe cap body, whereby calories conducted from the heat treatment boat tothe heat insulating unit and the cap body can be reduced also with theresult that occurrence of slips can be suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the heat treatment boat according to afirst embodiment of the present invention showing a general structurethereof.

FIG. 2 is a perspective view of the heat treatment boat of FIG. 1.

FIG. 3 is a plan view of the wafer support member of the heat treatmentboat of FIG. 1.

FIG. 4 is a partial vertical sectional view of the heat treatment boatof FIG. 1.

FIG. 5 is a plan view of a modification of the wafer support member ofFIG. 3.

FIG. 6 is a plan view of another variation of the wafer support memberof FIG. 3.

FIG. 7 is a schematic explanatory view (front view) of a wafer transfermode using the wafer support member of FIG. 6.

FIG. 8 is a vertical sectional view of the heat treatment boat accordingto a second embodiment of the present invention, which shows the heattreatment boat is loaded in a heat treatment furnace.

FIG. 9 is a perspective view of a major part of the heat treatment boatof FIG. 8.

FIG. 10 is a broken perspective view of another variation of the heattreatment boat of FIG. 8.

FIG. 11 is a perspective view of the heat treatment boat according to athird embodiment of the present invention showing a general structurethereof.

FIG. 12 is a vertical sectional view of the heat treatment boat of FIG.11, which shows the heat treatment boat is loaded in a heat treatmentfurnace.

FIG. 13 is a diagrammatic view explanatory of heat conduction in theheat treatment boat.

FIG. 14 is characteristic curves of slip occurrence rates for the thirdembodiment of the present invention and the prior art.

FIG. 15 is a perspective view of a modification of the heat treatmentboat of FIG. 11.

FIG. 16 is a vertical sectional view of another variation of the heattreatment boat of FIG. 11, which shows the heat treatment boat is loadedin a heat treatment furnace.

FIG. 17 is a perspective of further another variation of the supportmember of the heat treatment boat of FIG. 11.

FIG. 18 is a perspective view of the conventional heat treatment boat.

FIG. 19 is a partial enlarged vertical sectional view of a wafer supportmember of the conventional heat treatment boat.

FIG. 20 s a partial enlarged vertical sectional view of a wafer supportmember of the conventional heat treatment boat.

FIG. 21 is a vertical sectional view of conventional heat treatment boatloaded in a heat treatment furnace.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(First Embodiment)

The heat treatment boat according to a first embodiment of the presentinvention will be explained.

FIG. 1 is schematic respective view of a part of a vertical heattreatment apparatus including the heat treatment boat according to thefirst embodiment of the present invention. FIGS. 2 to 4 are views ofparts of the heat treatment boat of FIG. 1. The heat treatment boat 3according to this embodiment is used in heat treating disk-shapedwafers, objects to be treated. Hereinafter the heat treatment boat iscalled a "wafer boat", and an object-to-be-treated support member iscalled a wafer support member.

As shown in FIG. 1, the wafer boat 3 includes a circular top plate 31and a circular bottom plate 32 of, e.g., SiC, provided on the top andthe bottom thereof and opposed to each other, and four support rods41-44 of, e.g., SiC or polysilicon secured between the top and thebottom plates 31, 32. 150 sheets, for example, of wafer support members5 are provided parallel with each other at a certain interval betweenthe top and the bottom plates 31, 32. When the wafers W are formed aresilicon wafers, the support members 5 are formed of the same material asthe wafers, i.e., polysilicon.

Each of the support members 5 has an arcuate recess 5a of a sizesufficient to let in between, e.g., between the support arms 41, 42, atransfer arm 21 which will be described later, and has an inner diametersmaller than an outer diameter of the wafers W and is horseshoe-shaped.As shown in FIG. 4, a peripheral part of each support member 5 isinserted in four grooves 45 respectively formed in the support rods41-44 with a peripheral surface horizontally supported on the bottomsurface of the grooves 45 and the circumferential surface in contactwith the sides of the grooves 45. As also shown in FIG. 4, the supportmembers of the present invention have a thickness which, in combinationwith the space provided between the upper and lower surfaces of thegrooves, provides sufficient room for receipt of wafer W between theupper side of the support member and the upper surface of the groove.

A part of each wafer support member 5 located between the support rods43, 44 (a rear part when a part thereof through which a wafer W istransferred is called a front part) has an outwardly enlarged outerdiameter than the remaining part, and as shown in FIG. 3, a projectedpart 50 having a through-hole 51 formed in the central part is formedthereon. A fixation shaft 6 is passed through the through-holes 51 ofthe respective support members 5 for positioning stationary the wafersupport members 5 with respect to the support rods.

As shown in FIG. 2, the fixation shaft 6 has on the top a lockingportion 61 larger than the through-hole of the top plate 31 forprohibiting the shaft 6 from falling through the top plate 31. The lowerend of the shaft 6 is not secured to the bottom plate 32 but passedtherethrough, so that the shaft 6 can be pulled out of the top plate 31.The recesses 5a of the respective wafer support members 5 defineentrance spaces for a transfer mechanism, e.g., the transfer arm 21 toclear so as to transfer the wafers onto/from the wafer boat 3.

As shown in FIG. 1, the wafer boat 3 having the above-describedstructure is removably mounted on a heat insulating cylinder 2 having aflange 20 on the bottom. The heat insulating cylinder 2 is mounted on aboat elevator 22. A vertical furnace 7 is provided above the wafer boat3. Reference numeral 71 represents a gas feed pipe for supplyingrequired gases into a reaction tube in the vertical furnace 7, which isinvisible as viewed in the drawings. Reference numeral 72 indicates anexhaust pipe for exhausting the interior of the reaction tube.

Next the operation of the first embodiment will be explained. A wafer Wto be treated is put into the wafer boat 3 by the transfer arm 21through the recess 5a (the entrance spaces S) of a wafer support member5, is positioned right above the wafer support member 5, then thetransfer arm 21 is lowered relative to the wafer boat 3, and the wafer Wis transferred onto the wafer support member 5 of the wafer boat 3. Thewafer W is supported with a peripheral part thereof in surface contactwith the upper surface of the wafer support member 5.

Wafers W are transferred onto the wafer support members 5, e.g., up todown. When a set number of sheets of wafers W, e.g., 150 sheets havebeen loaded into the wafer boat 3, the boat elevator 22 is lifted toload the wafers W into the vertical furnace 7. When the wafers are heattreated at about 1200° C., the interior of the vertical furnace 7 hasbeen pre-heated up to about 800° C. and is heated up to about 1200° C.after the wafers W have been loaded, and a required heat treatment isconducted. When the heat treatment is over, the boat elevator 22 islowered to unload the wafers W out of the vertical furnace 7.Subsequently, the wafers W are dismounted from the wafer boat 3 in anoperation that is reverse to the above-described operation.

In the first embodiment, the respective wafers W are supported insurface contact with the large arcuate surface of the wafer supportmembers. Internal stresses applied to the wafers W are accordinglysmall, and even if parts of the outer peripheral portions of the wafersW should float from the wafer support members 5 because of sori, theremaining supported portion of the wafers W are still supported on theperipheral surfaces of the wafer support members without excessive loadsapplied to any one portion of the respective wafers as is in theconventional four-point support. As a result, occurrences of the slipscan be reduced. In the case that silicon wafers are used, since themelting point of silicon is 1410° C., the above-described structure isvery effective for heat treatments at above about 1000° C.

Because of the large support surfaces of the wafer support members 5,disuniformity of temperature distributions along the outer periphery ofthe wafers W can be reduced. Because the wafer support members 5 and thewafers W are formed of the same material, both have the same temperatureincreases and decreases in a heat treatment, and a temperaturedifference of the wafers W between the central parts and the peripheralparts is small. As a result thermal strain stresses can be considerablymitigated. That is, the wafers W and the wafer support members 5 havethe same radiant heat absorption and heat conductivity, and accordinglyheat deflections (temperature differences) between the parts of thewafers W in contact with the wafer support members 5 and the remainingpart of the wafers W can be suppressed, with a result that occurrencesof slips due to the heat deflections can be decreased.

The wafer support members 5 can be removed from the support rods, whichfacilitates the fabrication of the heat treatment boat. In the firstembodiment the wafer support members and the support rods may be formedin one-piece, but the one-piece formation limits their shapes. It isadvantageous that the wafer support members 5 are removable from thesupport rods.

Additionally the maintenance of the wafer support members 5 isfacilitated. For example, one of the wafer support members 5 is damaged,the fixation shaft 6 is pulled out to remove only the damaged wafersupport member and replace it with a new one. This is far moreadvantageous in comparison with a total displacement of the wafersupport members. In addition, as will be explained later, the wafersupport members can be of various types and may be displaced by thewafer support members of a type in accordance with a transfer method ofthe wafers.

Next, one example of variations of the heat treatment boat according tothe present invention will be explained. FIG. 5 shows a wafer supportmember involved in the example of variations. In this example, eachwafer support member 5 comprises two separate semi-arcuate members 5A,5B. The semi-arcuate members 5A, 5B are inserted into grooves of supportrods 41-44 and supported horizontal by their own weights. Fixationshafts 6A, 6B are inserted in through-holes 51A, 51B formed in therespective semi-arcuate members 5A, 5B. The two semi-arcuate members 5A,5B define an entrance space S therebetween for a transfer arm 21 whichis a transfer mechanism for transferring wafers W onto a wafer boat 3.

FIG. 6 shows another example of variations of the heat treatment boataccording to the present invention. In this example, each wafer supportmember 5 is in the form of a ring without recesses. Each wafer supportmember 5 has no entrance space S for a transfer arm 21 to clear, whichdisenables the use of the wafer transfer mode of FIG. 1. For thisreason, push-up means 8 as shown in FIG. 7 is disposed below a portstage 9.

As shown in FIG. 7, the push-up means 8 comprises a pole screw 83 whichis screw-engaged with a fixation table 81 and has a push-up portion 82,a motor 84 disposed on the fixation table 81, and a pulley 86 which isscrew-engaged with the pole screw 83 and is rotated by the motor 84 by abelt 85. In using such push-up means 8, the wafer boat 3 is disposed onthe port stage 9 which is independent from the boat elevator 22. A waferW to be treated is introduced inbetween the neighboring wafer supportmembers 5, 5 and is positioned right above the wafer support member 5.Then the motor 84 is driven to rotate the pole screw 83 to lift thepush-up portion 82 in the wafer boat 3, so that the wafer W on thetransfer arm 21 is pushed up from below. With the wafer W afloat, thetransfer arm 21 is drawn out, and the motor 84 is reversely rotated tolower the push-up portion 82 to transfer the wafer W onto the wafersupport member 5. Thus, a peripheral portion of the wafer W is supportedon the ring-shaped wafer support member 5 in surface contact with theupper surface of the wafer support member 5. Such transfer of the waferW is conducted sequentially, e.g., up to down, and when a set number ofsheets of wafers, e.g., 150 sheets have been mounted on the wafer boat3, the wafer boat 3 is transferred onto a boat elevator 22. Then theboat elevator 22 is lifted and loaded in a vertical furnace 7, and arequired heat treatment is conducted on the wafers. Then the boatelevator 22 is lowered, and the wafers W are unloaded from the verticalfurnace 7. Then an operation which is reverse to the above is conductedto take the wafer out of the wafer boat 3.

Thus, because the wafer support members 5 are removably mounted on thesupport rods 41-44, even in various wafer transfer modes, the wafer boatcomprising the top plate and the bottom plates 31, 32, and the supportrods 41-44 can be thus used as it is with only the wafer support memberssuitably replaced, whereby the wafer boat can be universally applicableto various wafer transfer modes.

In the above described embodiment, the wafer support members are formedof the same material as wafers, and the wafer boat includes the wafersupport members removably mounted on the plural support rods, but thewafer boat is not necessarily limited to this constitution. According tothe present invention, 1) the wafer support members are formed of thesame material as wafers, and the wafer support members are secured tothe support rods, and 2) the wafer support members are formed of adifferent material from the wafers, and the wafer support members areremovable from the support rods.

According to the first embodiment, the objects to be treated aresupported with peripheral parts thereof in surface contact with thearcuate or ring-shaped object-to-be-treated support members. As a resultinternal stresses of the objects to be treated are mitigated. Theobject-to-be-treated support members are formed of the same material asobjects to be treated. As a result, thermal strain stresses areconsiderably mitigated with a result of few occurrences of slips in theobjects to be treated.

According to the first embodiment, internal stresses of objects to betreated are mitigated, and slips of the objects to be treated can bedecreased. The object-to-be-treated support members are removable fromthe support rods. As a result the wafer boat can be easily fabricated,and when a part of the object-to-be-treated support members is damaged,the damaged one can be easily replaced with a new one. In addition theobject-to-be-treated support members can be replaced with those suitableto various wafer transfer modes.

(Second Embodiment)

Next a second embodiment of the present invention will be explained.FIG. 8 shows the heat treatment boat according to the second embodimentwhich is applied to an oxidation and diffusion furnace. In FIG. 8wafers, objects to be treated are loaded in a reaction tube. Thereaction tube 101 which is a heat treatment region has a double tubestructure of an inner tube 101a and an outer tube 101b which arerespectively arranged vertical and are secured (at a flange 113) on thebottom to a base plate not shown. The outer periphery of the lower endportion of the reaction tube 101 is surrounded by a heat insulatingmaterial 111. In the top of the inner tube 101a there are formed anumber of small holes 101c as gas passages. A gas feed pipe 151 isconnected to a side of the outer tube 101b. An exhaust pipe 152 isconnected to an inner tube 101a for exhausting the reaction tube 101.

A cylindrical liner tube 112 of, e.g., SiC surrounds the reaction tube101 on the outside thereof. A heater 110 surrounds the liner tube 112 onthe outside thereof. The heater 110 includes a coil of a resistanceheating line 110b inside of a heat insulating layer 110a.

Below the reaction tube 101 there is provided a cap body 131 forair-tightly closing the bottom opening of the reaction tube 101 whilewafers are loaded in the reaction tube 101. The cap body 131 is providedon a boat elevator 103 which is moved up and down by a lift mechanismnot shown. On the cap body 131 there is provided a heat insulatingcylinder 106 which provides a heat insulating unit. The heat insulatingcylinder 106 comprises a quartz cylindrical container 161 filled withquartz wool 162. As shown in FIGS. 8 and 9, a ring-shaped intermediatemember 107 of a material having high radiant heat absorption, e.g., SiCwhose upper diameter is larger than a lower diameter thereof is providedon the heat insulating cylinder 106 through a faucet joint portion 106A.A wafer boat 102 is mounted on the intermediate member 107 with a lowerprojection thereof engaged in the intermediate member 107. Theintermediate member 107 functions as a mounting member for the waferboat 102, and acts to suppress heat radiation from the lower part of thewafer boat 102 while heated. The intermediate member 107 has a heightof, e.g., 100-200 mm.

The wafer boat 102 of a material with a high radiant heat absorption,e.g., SiC. The wafer boat 102 comprises four, for example, support rods123 erected between a top plate 121 and a bottom plate 122, and grooves(not shown) formed in the respective support rods 123 for supporting 100sheets, for example, of wafers W horizontal at a certain verticalinterval.

The operation of the second embodiment will be explained. First theheater 110 is heated to heat the interior of the reaction tube 101through the liner tube 112 homogeneously up to, e.g., about 800° C. Then100 sheets, for example, of wafers W to be treated are mounted on thewafer boat 102 with the boat elevator 103 positioned below the reactiontube 101. Then the wafer boat 101 is lifted to a position where the capbody 131 closes the bottom opening of the reaction tube 101 (the stateof FIG. 8) to load the wafers W into the reaction tube 101. When thewafers W are heated up to the same temperature of the atmosphere in thereaction tube 101, the heater 110 is further intensified to raise thetemperature in the reaction tube 101 up to 1200° C. A processing gas isfed into the inner tube 101a from the gas feed pipe 151 through thesmall holes 101c while the reaction tube 101 is exhausted through theexhaust pipe 152 to maintain the interior of the reaction tube 101 undera set pressure, and a diffusion treatment, for example, is conducted onthe wafers W.

The wafer boat 102 and the intermediate member 107, which are formed ofSiC, quickly rise in temperature in the wafers W heating step, which, inthe second embodiment extends from the time of loading of the wafers Winto the reaction tube 101 until the wafers W are stabilized up to anatmospheric temperature in the reaction tube 101 (about 800° C.),However, the heat insulating cylinder 106, whose radiant heat absorptionis low, slowly raises its temperature. As a result, a temperaturedifference occurs due to a difference in the temperature rise betweenthe wafer boat 102 and the heat insulating cylinder 106. But thepresence of the intermediate member 107, whose temperature rise isquick, between the wafer boat 102 and the heat insulating cylinder 106suppresses temperature drops of a lower part of the wafer boat 102 dueto slower temperature rises of the heat insulating cylinder 106.

That is, an amount of radiation from the lower end of the intermediatemember 107 to the heat insulating cylinder 106 is considered large, butmaking a height (length) of the intermediate member 107 sufficient tokeep the upper end of the intermediate member 107 from the influence ofthe radiation from the lower end of the intermediate member 107 cansuppress radiation from the lower end of the wafer boat 102 to theintermediate member 107. Accordingly temperature drops of the lower partof the wafer boat 102 can be reduced. Temperature differences in theplane between the supported part of each wafer W (the support rod 123 ofthe wafer boat 102) and the rest part thereof can be small. Accordinglythermal strain stresses can be mitigated, and less slips occur. The useof the intermediate member 107 is very effective to suppress occurrenceof slips in heat treating wafers W of, e.g., 8 inches at above 1000° C.

In addition to this advantageous effect the second embodiment canprovide the following advantageous effect. Quartz lowers its strengthand tends to undergo thermal deformation at a high temperature of 1200°C. The mount member of quartz which is a part of the conventional heatinsulating unit is located in the region surrounded by the heater, andtends to be thermally deformed. This is a problem. But in the secondembodiment, the portion corresponding to the conventional mount memberis formed of SiC having good high temperature strength, and the heatinsulating cylinder can be kept from thermal deformation with a resultof a long lifetime.

The presence of the intermediate member, which quickly raises itstemperature, and is on the side of the lower end of the wafer boat,shortens a heat recovery time (a period of time from loading of thewafers into the pre-heated reaction tube to stabilization of the wafersat an atmospheric temperature ) of a part near the lower part of thewafer boat, which leads to high throughputs.

In the second embodiment, the material of the wafer boat and theintermediate member 107 is not limited to SiC, and may be anothermaterial, such as polysilicon as long as it has high radiant heatabsorption. The structure of the intermediate member 107 is not limitedto the above, and may include, as shown in FIG. 10, a plurality ofsupport rods, e.g., three support rods, erected on a disc body 171having the upper surface closed, and the support rods 172 are insertedin blind holes 122A in a bottom plate 122 so as to support the waferboat 102. The intermediate member 107 may be in one-piece with the waferboat 102.

According to the second embodiment, even when temperature differencesoccur due to differences in temperature rise speed between the heattreatment boat (wafer boat) and the heat insulating unit, theintermediate member of high radiant heat absoprtion is provided betweenthe heat treatment boat and the heat insulating unit and reducestemperature drops at the lower part of the heat treatment boat with theresult that occurrence of slips in objects to be treated can besuppressed.

(Third Embodiment)

The heat treatment boat according to a third embodiment of the presentinvention which is applied to an oxidation and diffusion furnace. Asshown in FIGS. 11 and 12, a wafer boat 202 used in the third embodimentincludes four support rods 221-224 which are planted vertically andcircumferentially in the upper surface of an annular support member 231,and which are secured at the upper ends by an annular fixation member232. Grooves 220 are formed in the respective support rods 221-224 at avertical interval for supporting peripheral parts of the undersides ofwafers W. Two (221, 222) of the support rods 221-224 support the wafersat a left and a right position on the side near to the entrance of thewafers into the wafer boat 202 by means of transfer arm 252. The othertwo support rods 223, 224 are positioned so as to support the wafers ata left and a right position on the side remote from the entrance of thewafers into the wafer boat 202. The support rods 221-224, and thesupport member 231 and the fixation member 232 are formed of, e.g., SiC,but their material is not necessarily limited to SiC and may be, e.g.,quartz.

The wafer boat 202 is disposed, through a heat insulating unit, a heatinsulating cylinder 204 of, e.g., quartz on a cap body 205 which is toclose a bottom opening of a reaction tube which will be explained later.The cap body 205 is disposed on a boat elevator 251. An annular stand241 is formed on the upper surface of the heat insulating cylinder 204,and the wafer boat 202 is disposed on the stand 241.

A vertical furnace is disposed above the wafer boat 202. The structureof the vertical furnace will be briefly explained with reference to FIG.12 showing the state in which the wafer boat 202 is loaded in thevertical furnace. A reaction tube 206 providing a heat treatment regionhas a double tube structure including an inner tube 261 and an outertube 262 and is secured to a base plate not shown at a flange 263 on thelower end. The outer peripheral surface of a lower end portion of thereaction tube 206 is covered by a heat insulating material 264. A gasfeed pipe 265 is connected to a side of the outer tube 262. An exhaustpipe 266 is connected to the inner tube 261 for exhausting the interiorof the reaction tube 206.

The reaction tube 206 is surrounded on the outside by a cylindricalliner tube 267 of, e.g., SiC. The liner tube 267 is surrounded on theoutside by a heater 268. The heater 268 comprises a coil of a resistanceheating line 268b provided inside a heat insulating layer 268a.

Here flow of heat in the wafer boat 202 will be explained. As seen inFIG. 12, the support rod 221 is heated by radiant heat of the heater 268through the liner tube 267. Since no heaters are present above and belowthe wafer boat 202, secondary radiation takes place from the upper andthe lower ends of the wafer boat 202, i.e., the fixation member 232 andthe support member 231, and the heat is radiated. But the fixationmember 232 and the support member 231 are annular, and their radiationareas are small.

The heat flowing in the loop of FIG. 13 (indicated by the arrows) fromthe wafers W to the support surfaces of the grooves 220 of the supportrods 221-224 to the upper end portions (lower end portions) of thesupport rods 221-224 is small in comparison with that of theconventional wafer boat having the solid top plate and bottom plates. Asa result, temperature differences between the upper and the lower endportions of the support rods 221-224, and the rest parts thereof aresmall. Temperature uniformity in plane of the wafers are so high thatthermal stresses in the peripheral parts of the wafers supported in thegrooves 220 are small, and occurrence of slips can be suppressed.

The advantageous effect of the third embodiment is shown in FIG. 14.Amounts of slips (a length of slips formed in one sheet of wafer) aretaken on the horizontal axis. Levels of the wafers in the wafer boat aretaken on the vertical axis. The solid line indicates results of thethird embodiment (W), and the dot line indicates results of theconventional wafer boat (C). As seen in FIG. 14, in the thirdembodiment, amounts of slips are smaller near the upper and the lowerend portions of the wafer boat in comparison with the conventional waferboat. The amounts of slips sharply decrease toward the center. Incontrast to this, in the conventional wafer boat (chain line) the curveof decreases of amounts of slips is blunt. As seen from these results,according to the third embodiment, occurrence of slips can be suppressedover a wide vertical range of the wafer boat, and throughput can beaccordingly high.

The third embodiment may have the structure of FIG. 15 for suppressingoccurrence of slips. In the example of FIG. 15, the wafer boat 202 has abottom plate 225 and a top plate 226 which are in the form of solidplates. Three, for example, members 271-273 of small horizontal sectionare disposed on the upper surface of the heat insulating cylinder 204 atcircumferentially trisected positions. The support members 171-173support the wafer boat 202 at three points. Also on the upper surface ofthe cap body 205, three support members 281-283 of small horizontalsection are disposed at circumferentially trisected positions. The threesupport members 281-283, which are inserted into holes 281A formed inthe bottom of the heat insulating cylinder 204, support the heatinsulating cylinder 204 at three points.

In this structure, heat is radiated by heat conduction from the bottomof the wafer boat 202 through the heat insulating cylinder 204 to thecap body 205. Against this heat flow, the support members 271-273, andthe support members 281-283 act as heat conduction suppressing regions,in other words, the heat conducting path is restricted, whereby thedownward heat radiation is suppressed. Calories escaping from theperipheral parts of the wafers W to the support rods 221-224 isaccordingly so small that thermal stresses in the peripheral parts ofthe wafers W are small, and occurrence of slips can be suppressed.

Wafers W were heated at 1200° C. for 5 hours by the heat treatmentapparatus using the wafer boat of this structure and the conventionalapparatus. This heating was conducted 3 times, and wafers W werereplaced by new ones for each heating. A total length of the slips inthe lowest wafers W in the conventional apparatus was about 25 mm, andthat of this embodiment was about 6 mm. It was confirmed that structureof this embodiment is superior.

This embodiment may have both the structure of FIG. 11 and that of FIG.15. A vertical heat treatment apparatus having both structures is shownin FIG. 16. In forming the heat conduction suppressing regions (in theabove-described embodiment the regions where the support members271-273; 281-283 are disposed) between the wafer boat 202 and the heatinsulating cylinder 204, and between the heat insulating cylinder 204and the cap body 205, the support is not necessarily the three-pointsupport and may be four-point support. It is preferred to minimize thearea of the support point to suppress heat conduction.

Such heat conduction suppressing region may be formed either between theheat insulating cylinder 204 and the wafer boat 202, or between the heatinsulating cylinder 204 and the cap body 205. In place of erecting thesupport members on the heat insulating cylinder 204 and the cap body205, a cylindrical intermediate member 209 as exemplified in FIG. 17 maybe separately prepared, and for restricting the heat path slits 290 areformed circumferentially in the middle of the side with three islands291-293 left.

What is claimed is:
 1. A heat treatment boat for mounting a number ofdisc-shaped objects to be treated at a vertical interval for heattreatment thereof in a vertical heat treatment furnace, the heattreatment boat comprising two semi-arcuate support members provided onsupport rods at a vertical interval for supporting the objects to betreated in surface contact with the undersides of peripheral parts ofthe objects to be treated.
 2. The heat treatment boat according to claim1, wherein the support members are removably mounted on the supportrods.
 3. The heat treatment boat according to claim 1, wherein thesupport members are formed of the same material as the object to betreated.
 4. The heat treatment boat according to claim 1, wherein thesupport members are formed of a different material from the object to betreated.
 5. The heat treatment boat according to claim 1 furthercomprising fixation shafts extending vertically with said support rods,and said support members each including an elongated outer diametersection with a through-hole formed therein through which a respectiveone of said fixation shafts extends.
 6. The heat treatment boataccording to claim 1 wherein said support rods each include verticallyspaced grooves, said grooves each having a lower support surface forcontact with an underside, peripheral portion of a respective one ofsaid support members and an upper surface vertically spaced from saidlower support surface, said grooves and said support members beingdimensioned and arranged such that a peripheral edge of one of the discshaped objects is positionable between an upperside of the one of saidsupport members and the upper surface of the groove supporting said oneof the disc shaped objects.
 7. The heat treatment boat according toclaim 1 wherein said semi-arcuate support members have ends spaced afixed distance apart with said distance being designed for receivingtherethrough a horizontally extending transfer arm.